Scientists had thought that the Gardiner’s Seychelles frog—at 11 millimeters among the tiniest in the world—was deaf because it doesn’t have a middle ear, a critical component of hearing that’s found in most mammals.

The middle ear contains three small bones called ossicles that connect the eardrum to the inner ear. When a sound wave hits the eardrum, it moves back and forth, causing the ossicles to move and send electric signals to the brain, which then registers the signals as sound.

Scientists knew of other frog species without middle ears that would still croak to each other. So they wondered: Can Gardiner’s frogs somehow hear?

To answer the question, a team set up loudspeakers in the frogs’ rain forest habitat of the Seychelles Islands, in the western Indian Ocean, and broadcasted recordings of frog calls. Surprisingly, the male frogs hopped closer to the loudspeaker and responded, indicating they’d heard the noise, said study co-author Thierry Aubin, a bioacoustics expert at the Université Paris Sud in France.

Hear Hear, Little Frogs

Next, the team had to identify how the frogs were hearing—that is, how the sound waves were getting to their brains. So the scientists x-rayed one of the tiny frogs. The images revealed that the frogs’ pulmonary system is poorly developed, suggesting that the lungs aren’t contributing to hearing, according to the study, published September 2 in the Proceedings of the National Academy of Sciences.

So the scientists refocused their experiments on the frogs’ heads. By making various 3-D simulations of how sound travels through the frogs’ heads, the scientists found that the bones in their mouths act as an amplifier for sound waves.

Further x-rays showed that sound travels from the mouth to the inner ear, thanks to an evolutionary adaptation: The frogs have thinner and fewer tissue layers between the mouth and inner ear.

“It’s interesting because this was never found before,” Aubin said.

The team also suspects that the frogs’ odd hearing strategy may be due to their long isolation. About 47 to 65 million years ago, the Seychelles broke off from the supercontinent Gondwana–one of two land masses comprising the supercontinent Pangaea–splitting the frogs off from other species. So hearing through the mouth may actually be a holdover from ancient life-forms, the authors say.